Optimization of acid strength and total organic carbon in acid processes (C-2644)

ABSTRACT

A method and system to optimize the acid strength and level of total organic carbon (TOC) of process streams in which specific hydrocarbons are extracted from hydrocarbon mixtures by sulfuric acid.

BACKGROUND OF THE INVENTION

The present invention is a system and method to control the acid contentand/or total organic carbon of a feed stream including an acid, waterand total organic carbon. The present invention permits on-lineprediction of acid and TOC composition in order to adjust acid purgerates or the addition of oxidants used to remove carbon.

The process is particularly important in sulfuric acid processes. Thecomposition of the sulfuric acid recycle stream in a sulfuric acidprocess is essentially comprised of sulfuric acid, water, and organiccarbon (TOC). The latter component is a class of heavy hydrocarbons, aportion of which may become insoluble in the sulfuric acid and depositin vessels, lines, or on heat exchange surfaces. Unit shutdown andmaintenance are required to clean the heat exchanger surfaces in theacid concentration section.

Many petrochemical processes, such as alkylation and alcohol production,involve the partially selective extraction of certain hydrocarbonspecies from a hydrocarbon mixture, by reacting the mixture with aqueousacid. For example, in isopropanol production, propylene is extractedfrom a hydrocarbon stream with aqueous sulfuric acid. Extraction occursdue to the formation of organic sulfates. Further processing may includehydrolysis to convert the sulfates to alcohol, dehydrogenation reactionsto convert the alcohol to ketone, and regeneration of the sulfuric acidfor recycle to the extraction process.

Economic advantages associated with minimizing energy and feed acidconsumption, and maximizing operability and product yield may berealized by monitoring and controlling the composition and properties ofthe streams associated with the various steps in the process. Forexample, a means to determine the acid strength would permit control ofthe addition of hydrolysis water, thereby optimizing alcohol production,while minimizing the energy expended to reconcentrate the acid forfurther use.

Another example is a method to determine the total organic content (TOC)of the process streams. This material includes higher molecular weighthydrocarbons formed in the sulfuric acid process, which becomesinsoluble in the process stream and deposits on reactor and pipe walls.A method to quantify the amount of this material could be used todetermine the optimum process condition.

The present invention is a spectroscopic method and system to determinethe acid strength and total organic carbon (TOC) content of acid processstreams. It is accurate, rapid, and precise and may be implementedon-line by the use of a fiber optic probe. The invention may be used todetermine the acid strength by the difference between the acid and waterand to indicate the TOC levels in the acid.

SUMMARY OF THE INVENTION

The present invention includes system and a method to determine the acidstrength and/or total organic carbon content of a stream which includesan acid, water and organic carbon. The method includes the steps ofilluminating the stream with optical radiation, determining thedifference in the optical absorptivity of the stream at two selectedwavelengths, and for determining the weight percent of the water and/ortotal organic carbon content from the difference in the opticalabsorptivity. We define absorptivity as the negative of the logarithm ofthe ratio of the transmitted light intensity to the incident lightintensity, the logarithm being divided by the path-length through theabsorbing material by the light. The absorbance is the absorptivitymultiplied by the pathlength.

In a preferred embodiment, the acid is sulfuric. If water content isbeing determined, then two selected wavelengths are about 1450 nm and1300 nm.

If organic carbon content is being determined then the two selectedwavelengths are about 546 nm and 820 nm.

For example, in isopropanol production, propylene is extracted from ahydrocarbon stream with aqueous sulfuric acid.

It is desirable to determine the total organic carbon (TOC) of theprocess streams. This material includes higher molecular weighthydrocarbons formed in the sulfuric acid processes, which becomesinsoluble in the process stream and deposits on reactor and pipe walls.A means to quantify the amount of this material could be used todetermine:

(1) the optimum rate of purging the acid from the process to achieve atarget TOC level in the acid inventory, and/or

(2) control the addition of oxidants which may be added to convert theseheavy carbon species to lighter species like CO, CO₂, or acetic acidwhich can be readily removed from the acid inventory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a parity plot of the water content of several acid feedstreams predicted by the present invention compared to the actual watercontent.

FIG. 2 shows a parity plot of the water content of several acid feedstreams predicted by the present invention compared to water contentdetermined by the Karl Fischer analysis.

FIG. 3 shows the results of ten repeat measurements of one sample.

FIG. 4 shows a graph of the organic carbon content of several samplesversus the absorptivity difference at wavelengths 546 nm and 820 nm.

FIG. 5 shows a schematic of the optimization of a process to produceisopropanol using sulfuric acid.

DESCRIPTION OF PREFERRED EMBODIMENT

In present invention is a system and method to determine the watercontent and/or total organic carbon content of an acid feed stream.

The difference in absorptivity at wavelengths of 1450 nm and 1300 nm canbe used to predict the water content with an accuracy of 0.8 wt % overthe range of 3-100 wt %, and with an accuracy of 0.13 wt % over the morenarrow range of 20-60 wt %. The precision was determined to be 0.02 wt %at a water content of 34 wt %.

Calibration samples of known mixtures of H₂ SO₄ and distilled water wereprepared, and their spectra measured in a 2 nm pathlength cuvette overthe wavelength range of 800 nm to 1600 nm in 1 nm increments. Thedifference in absorptivity at 1450 nm anti 1300 nm was used in aregression model in which a third order polynomial was regressed againstthe known water content of the samples. A parity plot of the waterfraction, predicted from the optical method, versus the actual watercontent is shown in FIG. 1.

The regression coefficients were used to predict the water content ofsix samples, whose water content was determined by Karl Fischeranalysis, and 17 known water or acid dilutions of these samples. Aparity plot of the spectrally-predicted versus the Karl Fischer resultsis shown in FIG. 2.

Ten repeat measurements were made on one sample and the averageabsorbance difference was found to be 0.9059 AU with a standard error of+/-0.00061 AU. This absorbance difference translates into a watercontent of 34.9 wt % and the standard deviation of the measurementstranslates into a measurement precision of +/-0.02%. The results of therepeat measurements are shown in FIG. 3.

In another aspect of this invention, the TOC can be correlated to thedifference in absorptivity at wavelengths of 546 nm and 820 nm to a TOCcontent of 2 wt %. Spectra of six samples were measured over thespectral region of 190 nm to 820 nm with increments of 2 nm. The TOClevels of the samples were determined by a standard laboratory method. Ascatter plot of the wt% TOC as measured versus the absorptivitydifference is shown in FIG. 4.

For a typical stream both the water and TOC can be determined bymeasurements of the absorbances at the above stated wavelengths over anoptical path length of 2 nm with no further dilution of the sample.

While the preferred wavelengths for the determination of water for thisapplication are 1450 nm and 1300 nm, other wavelengths may also be used.For example, the first wavelength in the difference model for water maybe chosen from 1150 nm to 1550 nm, but preferably in the ranges between1150 nm and 1250 nm or 1350 nm and 1550 nm, and still more preferablybetween 1400 nm and 1500 nm. The second wavelength may be chosen to beany convenient wavelength different from the first and preferably so asto make the absorbance difference greater than 0.002 AU.

The present invention also includes the optimization of processes inwhich it is necessary to know acid strength or total organic carbon.FIG. 5 shows a schematic of a process in which sulfuric acid is used toproduce isopropanol. Propylene is extracted from a hydrocarbonfeedstream (1) in one or more extraction reactors (2) by contact with anacid feed stream (3). The extract (4) is mixed with a hydrolysis waterstream (5) in the alcohol recovery section (6) and is split into a crudealcohol stream (7) and a diluted acid stream (8). The diluted acid isreconcentrated in an acid concentration section (9) and the concentratedacid stream (10) is recycled as the extraction section's (2) acid feedstream (3).

Spectroscopic acid analyzing may be applied at point (A) in the diluteacid stream (8) to determine the acid strength and control the valve(11) which adjusts the addition of hydrolysis water (5) to provide anoptimum acid strength in the alcohol recovery section (6).

Spectroscopic analyzing may also be applied at point (B) in theconcentrated acid stream (10) to determine the acid strength and totalorganic carbon and control the addition of fresh acid (low in TOC) bycontrolling valve (12) or the purging of acid by controlling valve (13)in order to provide the target TOC level and acid strength, or theaddition of oxidants to reduce/prevent the buildup of TOC.

What is claimed is:
 1. A method for controlling acid strength of analcohol production process having a recovery step wherein an extractfeed is separated into crude alcohol and diluted acid, the methodcomprising the steps of:(1) irradiating with near-infrared radiation, adiluted acid stream from a recovery section of the alcohol productionprocess in which a separation is made of the extract feed into crudealcohol and dilute acid streams; (2) determining the absorptivity ofsaid dilute acid stream at least one selected wavelength; (3)determining the acid strength from said absorptivity; and (4)controlling the amount of water to be added to the extract feed based onmeasured acid strength to achieve the acid strength for optimizingalcohol production.
 2. The method of claim 1 wherein said acid issulfuric acid.
 3. The method of claim 1 wherein said absorptivity isdetermined at two selected wavelengths which are about 1300 nm and 1450nm.
 4. A method for controlling acid strength and total organic carbonlevels in an alcohol production process which includes an acidconcentration step, the method comprising the steps of:(1) irradiating,with visible and near-IR radiation, acid after said acid concentrationstep (concentrated acid stream); (2) determining the absorptivities ofsaid acid after said acid concentration step at two selected wavelengthsin the visible and two selected wavelengths in the near-JR; (3)determining the acid strength and total organic carbon levels from saidabsorptivities; and (4) controlling the acid strength and total organiccarbon level by adding fresh acid, purging concentrated acid or addingoxidants.
 5. The method of claim 4 wherein said acid is sulfuric acid.6. The method of claim 4 wherein said two selected wavelengths are about1300 nm and 1450 nm and 546 nm and 820 nm, respectively.